1. Field of the Invention
This invention relates to apparatus and method for determining the rate of flow of a fluid by a magnetic flow meter that measures an electrical potential difference developed in the fluid as the fluid moves through a magnetic field.
2. Background Information
In most prior art in-line magnetic flow meters, the electrical potential difference developed in the fluid is sensed by a pair of electrodes contacting the liquid and spaced apart from each other by the diameter of a round flow sensing passage. A magnetic field generated orthogonal to both the line between the electrodes and the direction of flow through the sensing passage is provided by two coils of wire located on opposite sides of and outside of the passage. Sophisticated electronics are used to energize the magnetic field, to amplify the tiny flow-related signals generated, and to reject various noise and drift signal components which would otherwise degrade measurement accuracy. These meters are characterized by an unobstructed flow passage offering very low pressure drop and high tolerance to solids in the fluid, high measurement precision, high power consumption, and high cost.
In his U.S. Pat. No. 6,571,642, the disclosure of which is herein incorporated by reference, the inventor taught an in-line magnetic flow meter that generated higher flow voltages because of the provision of an extended voltage sensing path between the pair of electrodes and of more efficient utilization of the magnetic flux. This extended path was configured as a quasi-annulus formed by obstructing the central part of the fluid flow path with an axial streamlined body. The approach taught in U.S. Pat. No. 6,571,642 was optimized for instruments used for measuring moderate flow rates in which obstruction of the flow passage could be accepted in the interest of reducing cost and power consumption.
The teachings of U.S. Pat. No. 6,671,642 have not been applied to flow meters intended for measuring high flow rates, or for use in large pipes. In these cases the flow obstruction of the streamlined axial body taught in U.S. Pat. No. 6,671,642 causes formidable flow impedance.